The present invention relates to generally an ultrasonic transducer in which an piezoelectric or magnetostrictive ultrasonic transducer is driven at the natural frequency thereof or at a frequency substantially equal thereto, and more particularly an ultrasonic generator including a circuit for maintaining the constant amplitude of mechanical oscillation of the ultrasonic transducer and another circuit for ensuring the stable, dependable and efficient mechanical oscillations of the ultrasonic transducer and which generator is simple in construction, compact in size, light in weight and easy to manufacture.
The piezoelectric or magnetostrictive ultrasonic transducers are widely used for converting the electrical oscillation into mechanical oscillations so that the ultrasonic waves may be used for various purposes such as cleaning, welding, atomizing of liquid and so on. In some ultrasonic generators, in order to amplify the mechanical oscillations of the ultrasonic transducer, a horn is attached thereto so that the mechanical oscillations with a higher amplitude produced at the free end of the horn may be used for atomizing liquid fuel or welding.
The prior art liquid fuel combustion devices incorporating the ultrasonic generator with a horn for atomizing the liquid fuel present the following problems: Firstly, in order to ensure the efficient operation of the ultrasonic transducer having a high quality factor Q, the transducer must be driven at the natural frequency thereof or at a frequency substantially equal thereto, but the natural frequency is dependent upon the shape and dimensions of the transducer and changes with the temperature variation. Therefore the oscillator for driving the ultrasonic transducer must oscillate at a frequency equal to the natural frequency of the transducer or at a frequency substantially equal thereto. Secondly, when the ultrasonic generator is used for atomizing the liquid, the amplitude of mechanical oscillations of the transducer must be such that the liquid may be atomized into particles having substantially the same particle size. If the amplitude is increased excessively, cavitation occurs, resulting in the larger particle sizes. On the other hand, when the amplitude is small, the atomization of the liquid cannot be satisfactorily attained. Thirdly, at the initial stage of the atomization, the driving current supplied to the transducer must be higher than in the steady state because a relatively large amount of the liquid on the atomizing surface at the start of the atomization cannot be satisfactorily atomized unless the amplitude of mechanical oscillations of the atomizing surface is considerably greater than that in the steady state. This phenomenon shall be referred to as the "hysteresis phenomenon" in this specification. This hysteresis phenomenon inevitably occurs in the liquid fuel combustion devices in which the liquid fuel is atomized into very small particle sizes in order to improve the combustion efficiency.
In order to overcome the problems described above, there has been proposed an ultrasonic generator to be described hereinafter with reference to FIG. 1, but unless the ultrasonic transducer thereof is driven at the natural frequency thereof or at a frequency very close thereto, the amplitude of mechanical oscillations sufficient for atomization of liquid cannot be obtained. Therefore, the natural frequency must be automatically detected to solve the first problem. Since the quality factor Q of the ultrasonic dynamic transducer is considerably high at its resonant frequency, the impedance is extremely small at the resonant frequency or at a frequency very close thereto so that when the voltage is applied to the transducer, the maximum current is obtained at the natural or resonant frequency. Therefore, the first problem may be overcome by the positive feedback of the voltage representative of the driving current flowing through the transducer. The second problem is to maintain constant the amplitude of mechanical oscillations of the ultrasonic transducer. To solve this problem, it is required to detect the amplitude by some suitable means in order to attain the feedback of the amplitude. In general, the amplitude of mechanical oscillation of the ultrasonic transducer is in proportion to the current flowing therethrough. Therefore, in the above prior art generator, the current flowing through the transducer is detected to change the voltage supplied from the electrical power source, thereby maintaining constant the current flowing through the transducer and consequently the amplitude of mechanical oscillations thereof. When a single-ended push-pull output-transformerless type Class B amplifier circuit is used, the current flowing therethrough is equal to that flowing through the ultrasonic transducer. Therefore, the transducer may be driven by a constant current supplied from a constant current supply circuit or current regulator without changing the voltage of the power source.
The third problem is to increase the magnitude of the driving current to be applied to the ultrasonic transducer for a predetermined time after the start of the liquid atomization. This problem may be solved in a simple manner by the combination of a delay circuit and two-level current regulators.